Melt-blowing techniques for forming very small diameter fibers, sometimes referred to as melt-blown fibers or microfibers, from thermoplastic resins are well known in the art. For example, the production of fibers by melt-blowing is described in an article entitled "Superfine Thermoplastic Fibers", appearing in Industrial and Engineering Chemistry, Vol. 48, No. 8, pages 1342-1346. This article describes work done at the Naval Research Laboratories in Washington, D.C.
Another publication dealing with melt-blowing is Naval Research Laboratory Report 111437, dated Apr. 15, 1954. Generally, melt-blowing techniques include heating a thermoplastic fiber-forming resin to a molten state and extruding the molten resin through a plurality of linearly arranged small diameter capillaries that are formed in a die as molten threads. The molten threads exit the die into a high velocity stream of gas, usually air, which may be maintained at an elevated temperature and which serves to attenuate the threads of molten resin to form fibers or, depending upon the degree of attenuation, microfibers having diameters less than the diameters of the capillaries of the die arrangement.
U.S. Pat. No. 3,849,241 to Butin, the disclosure of which is hereby incorporated by reference, discloses the manufacture of nonwoven mats by melt-blowing and describes, at column 4, lines 57 et seq., the formation of melt-blown fibers having diameters of from about 0.5 microns to about 400 microns by extruding degraded fiber-forming molten thermoplastic polymer resins as molten threads into an attenuating gas stream. Also disclosed is the fact that the diameters of the attenuated fibers will decrease as the flow of the attenuating gas through the gas outlets, which are located on either side of the die capillaries, increases. It is also stated that, at low to moderate attenuating gas velocities, the extruded molten threads, even after attenuation by the gas into fibers, remain essentially continuous with little or no fiber breakage and that fibers produced in such an arrangement have diameters of, preferably, from about 8 microns to 50 microns. Prior to extrusion the fiber-forming thermoplastic polymer resins are subjected to controlled thermal and oxidative degradation at temperatures ranging from about 550.degree. F. to about 900.degree. F., that is, from about 288.degree. C. to about 482.degree. C., preferably from about 600.degree. F. to 750.degree. F., that is , from about 316.degree. C. to about 399.degree. C., to effect a requisite degradation of the resin to reduce the viscosity of the fiber-forming resin. Typical fiber-forming thermoplastic resins are listed at column 4, lines 35 et seq. and commercially useful resin throughput rates are stated to be from about 0.07 to 5 grams per minute per die extrusion capillary, preferably at least 1 gram per minute per die extrusion capillary.
Some thermoplastic polymers are amenable to the melt-blowing process, while others do not seem to be. As is reported in Tappi, The Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, pages 74-77 (April 1973), a good deal of work has been done with the melt-blowing of polypropylene. Nonwoven webs made from polypropylene microfibers are said to be soft and drapeable. Other polymers that have been evaluated for use in melt-blowing include polyethylene, nylon 6, nylon 11, certain polycarbonates, polyesters of the kind useful as hot-melt adhesives, polyethylene terephthalate, poly(4-methylpentene-1), poly(tetramethylene terephthalate), and polystyrene. The Tappi article reports the properties of nonwoven webs made from melt-blown fibers of these materials. The article also reports that attempts have been made to melt-blow polysulfone, nylon 4, and polyphenylene oxide, without achieving acceptable operation in short screening trials.
The Tappi article also reports that blends of certain resins can be made up for melt-blowing. Thus, pellets of polypropylene and nylon 6 were mixed together, then melt-blown, to produce webs made from blends of these two materials.
In Textile World, February 1979, pages 83-84, there is a list of polymers that have been melt-blown successfully. That list appears to have been copied from the list in the Tappi publication, despite the passage of six years between the two respective publications.
For quite some time those in the art have been attempting to form elastomeric resins into fibrous nonwoven elastomeric webs. In fact, the prior art reveals that experimentation with polystyrene/poly(ethylenebutylene)/polystyrene elastomeric copolymer resins materials has occurred. See, for example, U.S. Pat. No. 4,323,534 to des Marais. Unfortunately, the physical properties of the products obtained by the des Marais process, for example, a nonwoven mat of melt-blown fibers, were apparently unsatisfactory because of the use of a leaching step. See also Jones, U.S. Pat. No. 4,355,425.